There is becoming an ever increasing demand for very high quality alternating current power, sometimes in very high power ratings, for such purposes as telephony and telecommunications, computers, etc. Very often, the quality of the voltage or power wave form of power derived from an ordinary commercial power source such as a local hydro-electric power authority or power generation company, is so poor as to create difficulties for the user; and such commercial power sources are likely to undergo voltage swings as other users come on or off the line. In any circumstances where a device is operating which is handling data at very high speeds, such as a computer, such poor quality commercial power may be intolerable. In such cases, elaborate power supply arrangements may have to be made, including the use of inverter circuits with ferroresonant voltage regulators in their output. Inverter circuits of the sort which may be particularly useful are taught in applicant's co-pending application Ser. No. 210,373 filed on Sept. 30, 1974; and a particularly suitable ferroresonant voltage regulator is taught in applicant's U.S. Pat. No. 3,824,449 issued July 16, 1974. In any event, it has very often been necessary to provide such power supply circuitry as mentioned above. Normally when an SCR circuit is being used, the gates of the SCR's are fired from a pulse-firing power oscillator which produces a series of sequential firing pulses for firing each of the SCR's in series. It may be that several SCR's will be fired at one time, but in any event at least two pulses are provided from the output of the oscillator for each cycle thereof, so as to alternately fire each of a pair of SCR's in known fashion. Usually in the past, the oscillators which have been used to control the firing of the SCR's by providing the firing pulses therefor have been uni-junction relaxation oscillators, transistor/IC hybrid oscillators, Schmitt triggers, tuning fork controlled oscillators, crystal controlled oscillators, blocking oscillators, etc.
However, difficulties which are inherent with the use of such oscillators as enumerated above include the requirement for very elaborate electronic circuitry to adapt the oscillators to SCR-firing circuitry. Even more elaborate circuitry is required, usually without success, to phase lock a plurality of SCR-type inverters in parallel, or to synchronize one or more SCR-type inverters to a commercial power line for frequency synchronization. In addition, most of the types of oscillators noted above are subject to RF interference, so that in the presence of RF signals the operation of the oscillator may be effected.
The present invention provides a pulse-firing power oscillator which, when free running, is self-starting and has voltage dependent frequency output. That is, the pulse-firing power oscillator of this invention, when free running, operates in a given frequency range which is predetermined by the design of the pulse transformer discussed in greater detail hereafter, but the precise output frequency of the oscillator is dependent upon the impressed DC voltage across it. The oscillator may comprise a number of different types of solid state components, usually transistors although integrated circuits, uni-junction transistors, SCR's, etc. may be used. The present pulse-firing power oscillator comprises a pulse transformer having an iron core, with a centre-tapped primary winding thereon and as many secondary windings as there are firing pulses to be sequentially produced in a series of pulses, with a half-wave rectifier in series with each secondary winding. At least a pair of back-to-back transistors are provided in a symmetrical circuit with one transistor connected at each end of the primary winding of the pulse transformer, the base bias voltage for that transistor being provided from the other end of the primary winding. A DC voltage is impressed across the circuit between the centre-tap of the primary winding and a common connection between the transistors which is further away from the primary winding than the bases of the transistors which are biased therefrom.
Because the oscillator frequency, when free running, is strictly dependent on the impressed DC input voltage, and the oscillator operates at relatively high power levels to produce output pulses capable of satisfying SCR gating characteristics, the oscillator output is immune to the usual RF interference normally found in power conversion circuitry.
It is sometimes necessary to run several inverters in parallel, or to synchronize one or more parallel-connected inverters to a source of commercial power for frequency standardization purposes. The reasons may be, in the case of parallel operation of inverters, the requirement for more power than a single inverter can supply; and in the case of synchronization of an inverter to a commercial power supply, so as to provide a clock synchronized to the commercial power system for time-accuracy purposes such as clocks. The latter requirements come from the fact that over a given period of time, commercial power frequencies are standardized so that a given number of cycles occurs in the period of, say, one month; and this may be important for billing or time-base purposes.
In any event, the pulse-firing power oscillator of the present invention may be easily synchronized to a commercial power line by utilizing a separate, auxiliary primary winding on the core of the pulse transformer. The auxiliary winding may be connected to the commercial power line and the frequency of the oscillator will become slaved to the commercial power line even though the power consumption of the oscillator from the commercial power line may be very low. This comes as a result of a forced minority hysteresis on the hysteresis characteristic of the core of the pulse transformer, as discussed in greater detail hereafter. Of course, through the use of auxiliary independent primary windings, a number of oscillators according to this invention may be operated in parallel so that the SCR-type inverters which they control will operate in parallel.
The pulse-firing power oscillator of the present invention may also be synchronized to another power source by inserting a synchronization signal derived therefrom to the oscillator circuitry -- usually, across the base-emitter junction of one or more of the switching transistors. The synchronization signal itself may be isolated or pre-conditioned through suitable wave-shaping, filter or isolating circuits.
Thus, pulse-firing power oscillators of the present invention may be connected with several identical oscillators in parallel for purposes of circuit redundancy with parallel-operated inverters. Also, any oscillator according to this invention may be frequency slaved to any frequency source having a cyclical output, regardless of its precise waveform; so that oscillators may be synchronized to extremely high accuracy frequency sources such as quartz crystal or atomic oscillators. Alternatively, pulse-firing power oscillators of the present invention may be feedback loop controlled, by sampling the oscillator frequency and by applying error correction through appropriate circuit means.